1. Field of the Invention
This invention relates to the corrosion protection of steel tank car containers or pressure vessels, as well as other vessels utilizing polyurethane foam insulation and more particularly, to the protection of the outer surface of the tank car container or pressure vessel and the inside surface of the outer shell which encapulates the container. The method of this invention includes preparing the outer surface of a steel container or pressure vessel and the inner surface of the outer shell by techniques known to those skilled in the art and applying a coating of corrosion-inhibiting material such as a lead and chrome-free polyamide epoxy to prevent pitting and corrosion of the outer and inner surfaces, respectively. The polyamide coating is compatible with polyurethane foam insulation which is commonly used to insulate tank cars and other vessels and is designed to inhibit corrosion resulting from the interaction of water vapor with certain constituents of the polyurethane foam material during operation of the tank car. The protective coating can be utilized regardless of the tank car service and regardless of the thickness or density of the polyurethane insulation located between the pressure vessel or container wall and the outer shell.
2. Description of the Prior Art
Department of transporation specifications for pressure tank car tanks (classes DOT-105, 109, 112 and 114) require that the exterior surface of a carbon steel tank or container and the inside surface of a carbon steel jacket or shell must be given a protective coating, except "that a protective coating is not required when foam-in-place insulation that adheres to the tank or jacket is applied". Various coating methods for protecting tank cars are known to those skilled in the art. U.S. Pat. No. 3,110,610, dated Nov. 12, 1963, to R. H. Goodell, describes a "Coating Method" for coating the inside surface of a tank car container or pressure vessel. The method includes pre-drying the interior of the tank with warm gases for a period of two to six hours and subsequently applying a series of four to eight liquid resin coatings, drying each coat with warm gases to a non-tacky state and baking the interior with warm gases for a period of from six to twenty hours. Various techniques for foaming the annulus between the outer surface of a pressure vessel or container such as a tank car container and the inner surface of the outer skin or shell are also known to those skilled in the art. U.S. Pat. No. 3,687,087, dated Aug. 29, 1972, to Frank R. Yurkoski discloses an "Insulating Structure for Interior of Railway Freight Cars". The disclosed insulating structure includes a layer of polyurethane foam material having its outer surface secured to the inner surface of the metallic car body and an elastomeric inner liner secured to the inner surface of the polyurethane foam material. The structure is designed to compensate for unusually high local stresses such as those realized from a tool or a workman within the car, with the intermediate foam layer deformed permanently while the elastomeric inner layer remains undamaged. An "Insulated Railway Tank Car and Method of Forming Same" is disclosed in U.S. Pat. No. 3,547,047, to Robert F. Needham. This patent includes a plurality of circumferential dividers secured to the inner container before the inner container is telescoped within an outer shell. After positioning of the inner container within the outer shell, the dividers form a plurality of compartments which are subsequently filled with a foamed-in-place, cellular insulating material. U.S. Pat. No. 3,876,739, dated Apr. 8, 1975, to Arthur F. Loveland discloses a "Method of Applying a Rigid Polyurethane Foam to the Exterior of a Railway Car". In this method a plastic or metal sheet is first applied to the interior of a mold and the mold is then secured to the car and is spaced about three inches from the sides of the car. A foamed cellular plastic such as a polyurethane foam is introduced into the space or cavity between the mold and the outer surface of the side sheets. After the foaming reaction has taken place and the foam has filled the cavity, the mold may be removed and the car moved to an adjacent area for foaming. The outer surface of the car thus foamed is even and smooth to facilitate painting and stenciling.
In light of the Department of Transportation regulations for protecting tank car containers or pressure vessels and the known art for applying insulating foam to the tank cars, applicant is unaware of any requirements for applying a protective coating to the outer surface of a container or pressure vessel or to the inside surface of the cooperating skin or outer shell, when foamed-in-place insulation is to be applied. However, it has surprisingly been found that pitting and corrosion of the outside surfaces of inner tank car containers or pressure vessels, as well as the inside surfaces of the outer shells has occurred, even under circumstances where polyurethane foam has been applied to the pressure vessel or container. Since this insulation contains Freon 11, Freon 12, a fire retardant known as trichloro propyl phosphate and other chemicals, certain reactions take place when water enters the insulation. Water frequently enters the insulation by way of condensation on the steel surfaces prior to insulation and during use of the tank car, particularly under circumstances where the tank car is refrigeratedd. Water which migrates into the insulation during use of the car reacts with the freon 11 (CCl.sub.3 F), freon 12 (CCl.sub.2 F.sub.2) and/or the trichloro-propyl phosphate (PCl.sub.3 C.sub.3 H.sub.7) to form hydrochloric acid (HCl), hydrofluoric acid (HF), phosphoric acid (H.sub.3 PO.sub.4) and/or carbonic acid (H.sub.2 CO.sub.3). Accordingly, the hydrochloric acid, hydrofluoric acid, phosphoric acid and carbonic acid which are formed as a result of this water migration and reaction of the water molecules with the Freon 11, Freon 12 and the fire retardant trichloro propyl phosphate outer metal skin of the pressure vessel, react with the carbon steel container and shell to cause pitting and corrosion of the container and shell. The most vigorous attack on the carbon steel surfaces is realized as a result of the formation of hydrochloric acid. This acid attack explains the corrosion and pitting of the carbon steel outer container surface and inner shell surface when the eleven gauge steel shell and foamed-in-place insulation is removed from a vessel which has been in service for a number of years. The corrosion and pitting are most prevalent near the bottom of the container and shell, because as the car moves along the track, vibration shifts the condensed water and acid to the bottom of the tank, thus causing a higher liquid concentration in the bottom of the insulated annulus formed by the outer surface of the container and the inner surface of the shell.
The solution to this problem lies in the application of a protective coating to the exterior surface of the pressure vessel or tank car container and optionally, to the interior surface of the outer shell prior to application of a polyurethane insulating foam in the annulus between the pressure vessel or container and the outer shell. It is an object of this invention to provide a protective coating on the outer surface of a carbon steel vessel, pressure vessel or tank car container which is provided with polyurethane insulation, in order to prevent corrosion and pitting of the outer surface.
Another object of this invention is to provide a method of applying a protective coating to the exterior surface of a polyurethane insulated steel vessel, tank car container or pressure vessel and the inner surface of a protective steel liner or shell disposed around the vessel container or pressure vessel to protect the protective liner and the vessel, pressure vessel or container from pitting and corrosion.
Another object of this invention is to provide a protective coating for the outer surface of steel vessels of various description, including tank car pressure vessels or containers and the interior surface of cooperating steel liners or shells in advance of application of polyurethane insulation, in order to minimize or prevent the pitting and corrosion of the outer and interior surfaces, respectively, due to the reaction of water vapor with certain ingredients in the polyurethane insulation.
Still another object of this invention is to provide a method for protecting the outer surface of a carbon steel pressure vessel or tank car container which is insulated with polyurethane foam, which method includes the steps of preparing the outer surface and applying a protective coating to the outer surface in advance of applying polyurethane insulation to the outer surface.
A still further object of the invention is to provide a method for protecting the outer surface of a steel pressure vessel or tank car container and the inner surface of an outer liner or shell encapsulating the pressure vessel, which method includes the steps of sandblasting or otherwise preparing the outer surface of the pressure vessel and the inner surface of the shell and applying a protective coating to the outer and inner surfaces, respectively, in advance of applying foaming polyurethane insulation in the annulus between the outer surface of the container and the inner surface of the shell.